You might think that these are all online, but they are not! Here are the briefs research guides I use:
1) Tried and true, if you are near a library with this print set: Gerhard Casper and Kathleen M. Sullivan, eds., “Landmark Briefs and Arguments of the Supreme Court of the United States: Constitutional Law”
3) Official U.S. Supreme Court website has several links to sources of briefs.
4) Guides: LLRX Supreme Court Web Guide (including a section on briefs)
There are other guides, but these are the ones that have worked well for me in the past.